1. Field of the Invention
The present invention relates to a piezoelectric transducer using a piezoelectric oscillating element as its driving means, and in particular relates to an improvement of such a piezoelectric transducer which is suitable for use as a piezoelectric loudspeaker, a piezoelectric microphone, a piezoelectric buzzer, and so on.
2. Description of the Prior Art
Conventionally, a piezoelectric transducer of this type has had a structure such as shown in longitudinal sectional view in FIG. 20 of the accompanying drawings.
Specifically, in this structure, a step portion 3 is formed at a longitudinally central portion of the interior of a tubular case 1 which has an open end, and a piezoelectric oscillating assembly 9 which is formed by adhering a piezoelectric oscillating element 5 (made of a per se known type of piezoelectric material) onto a surface of an electroconductive plate 7 of a circular shape is attached by its circular edge portion to the step portion 3 with elastic adhesive 11. Further, a sound emitting hole 13 is formed in the end surface of the case 1 which is not open, and a circuit board 15 having a drive circuit (which is not shown in the drawing) for driving the piezoelectric oscillating element 5 is mounted in the open end surface of the case 1, with wires which are also not shown in the figure being provided for electrically connecting the piezoelectric oscillating element 5 to said circuit board 15.
According to such a structure for a piezoelectric electro-acoustic transducer, when the piezoelectric oscillating element 5 is driven by the drive circuit, the piezoelectric oscillating assembly 9 is caused to oscillate by way of the oscillation of the piezoelectric oscillating element 5, and this causes the production of sound in the air filling the chamber 17 defined on the side of the piezoelectric oscillating assembly 9 towards the closed end of the casing 1 and the sound emitting hole 13, and this sound is thence emitted to the outside mainly through the sound emitting hole 13. Such a sound has frequency characteristics in which the sound level is high near the characteristic resonance frequency A of the piezoelectric oscillating assembly 9 and also near the characteristic resonance freqency B of the acoustic space or chamber 17. Such a frequency characteristic is exemplarily shown in FIG. 21 of the accompanying drawings as a graph of sound intensity against frequency.
Because the characterisic resonance frequency B may be changed by varying the shape and the volume of the acoustic space 17 by adjusting the shape of the case 1 and of the sound emitting hole 13, thus by bring the resonance frequency B of the chamber 17 near to the characteristic resonance frequency A of the piezoelectric oscillating assembly 9 it is conventionally considered to be possible to broaden the frequency range of high sound pressure level.
However, according to such a structure for a piezoelectric transducer, the adjustable factors are limited to the shape and the dimensions of the piezoelectric oscillating assembly 9 and of the case 1, and the characteristic resonance frequencies A and B are relatively steep and are few in number (i.e., two), and thus, even when the characteristic resonance frequency B is varied, it is not possible to broaden the frequency range of high sound pressure level, and further it is difficult to obtain a favorable sound pressure level over a wide frequency range.
Therefore, such a piezoelectric transducer is suitable for driving air, i.e. for producing a sound, at a certain substantially constant frequency, as in the case of a piezoelectric buzzer, but when it is to be driven by a signal the frequency of which varies over a wide range, as in the case of a loudspeaker, it is difficult to obtain a favorable sound pressure and to get crisp reproduction over a wide frequency range, and the reproduced sound tends to have a squeaky tone.
Moreover, because such a piezoelectric transducer has the above described structure in which the piezoelectric oscillating assembly 9 is secured within the tubular case 1, it is hard to make its configuration compact, and in particular it is hard to make said structure in particular low profiled (by which is meant short in longitudinal extent), while broadening its frequency range at the same time.